The invention relates to improvements in an electrodeless low-pressure discharge lamp comprising a discharge vessel sealed in a vacuum-tight manner and having a discharge space containing ionizable vapor and rare gas, and the discharge vessel having an inwardly extending protuberance for receiving a body of soft magnetic material cooperating with an electrical coil surrounding the magnetic body.
Such a lamp is known from GB No. 2 133 612 A and corresponding U.S. Pat. No. 4,568,859.
The known lamp is a low-pressure mercury discharge lamp. Low-pressure mercury discharge lamps have a comparatively low operating temperature. An optimum efficiency is attained if the lowest temperature of the discharge is about 40.degree.-90.degree. C. An attractive property of the known electrodeless lamp is that the discharge vessel has only small dimensions as compared with lamps having electrodes and are not constrained to have a tubular elongate discharge vessel. The light produced by a compact lamp, such as the known electrodeless lamp, can readily be concentrated by a luminaire.
Like low-pressure mercury discharge lamps having electrodes, low-pressure sodium discharge lamps have an elongate tubular discharge vessel. Also in these sodium lamps, a compact lamp vessel would be advantageous.
However, low-pressure sodium lamps have an optimum efficiency at a comparatively high operating temperature. The lowest temperature of the discharge vessel is then about 260.degree. C.
In order to attain this comparatively high minimum temperature, the discharge vessel in the conventional low-pressure sodium discharge lamps provided with electrodes is arranged inside an evacuated outer bulb.
Soft magnetic materials, such as ferrites, have a low resistance to heat. With increasing temperature, the specific magnetic losses increase, while at an elevated temperature moreover the magnetic permeability of the materials starts to decrease. As a result, the efficiency of electrodeless lamps containing said materials is lower.
For low-pressure sodium discharge lamps, which cannot be equalled by any other lamp type from a view-point of efficient conversion of electrical energy into visible radiation, and for other lamps containing an ionizable vapor at a comparatively low vapor pressure, such as metal halide, for example AlCl.sub.3, SnCl.sub.2, there are consequently factors contrasting with each other. In order that the light generated by a lamp can be fully utilized by a luminaire cooperating with said lamp, the lamp has to be compact. An electrodeless lamp is very suitable for this purpose. For a high efficiency, the discharge vessel has to be surrounded by an outer bulb in order to thermally isolate the discharge. In the other hand, a body of soft magnetic material in an electrodeless low-pressure discharge lamp is already thermally heavily loaded and this thermal load becomes even higher if the lamp is surrounded by an outer bulb and is consequently thermally isolated from the environment.